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First-principles thermodynamic calculations were carried out at the interface level for understanding the precipitation of coherent L12-phase nano-structures in dilute Al-Sc-Er alloys.All energetics,relevant to bulk substitution,interface forma-tion,interfacial coherent strain and segregation,were calculated and used to evaluate the nucleation and relative stabilities of various possible L12 nano-structures.Only matrix-dissolved solute Er(or Sc)can substitute Sc(or Er)in L12-Al3Sc(or Al3Er).The inter-substitution between L12-Al3Sc and Al3Er is not energy feasible.Ternary L12-Al3(ErxSc1-x)precipitates tend to form the Al3Er-core and Al3Sc-shell structure with a sharp core/shell interface.Three possible formation mechanisms were proposed and examined.The effects of Er/Sc ratio and aging temperature on the relative stabilities of L12-phase nano-structures in Al were also discussed.